This invention relates to systems for projecting colored images and, more particularly, to an improved system for large screen display of electronic video information such as high definition television.
The projection of color images, for example color television pictures, has been accomplished by systems employing various combinations of electric and optical equipment. In a system described in U.S. Pat. No. 2,813,146, dated Nov. 12, 1957, color images are produced by a deforming light modulating medium, for example a deformable oil film, as it is scanned by an electron beam to provide diffraction gratings thereon containing intelligence with respect to the color components, so that light projected through the modulating medium and through a suitable light masking system conveys both the intensity variation and the color selection to the screen in point-by-point correspondence with the scene to be reproduced.
High definition television (HDTV) systems are being developed in countries around the world, bringing attention to the need for the development of larger screen devices suitable for the display of these high quality pictures. The recent development of active matrix liquid crystal displays (AMLCD) using thin film transistors (TFT) as switching elements have made such devices attractive for use in light-valve projectors. Projection systems for NTSC video using TFT-switched AMLCDs are already on the market and an HDTV projector using such LCDs is described in an article entitled "A 750-TV-Line-Resolution Projector Using 1.5 Megapixel a-Si TFT LC Modules", by K. Kazuhiko, et al., SID91 DIGEST, pp. 415-418, and details of the LC modules used in the system are described in a companion paper by Y. Okita, et al. appearing on pp. 411-414 of the same journal entitled "A 1.5-Megapixel a-Si TFT-LCD Module for HDTV Projector". The structure of the optical system of the projector described in these articles is shown in FIG. 1.
Referring to FIG. 1, light from a source of white light, such as a metal halide lamp 10, is formed into parallel optical beams by a dichroic reflector 12. Two dichroic mirrors 14 and 16 split the beams into red, blue and green beams and directs them through respective condenser lenses to respectively illuminate a red LC panel 18, a blue LC panel 20 and a green LC panel 22, each of which has a size of 67.5 mm.times.120 mm and about 1.5 million pixels (1024.times.1440). As each beam passes through its respective LC panel its transmittance is modulated in accordance with a processed video signal and synch pulses applied to the active matrix. The modulated beams are then recombined by two dichroic mirrors 24 and 26, and the combined beam is projected onto a large screen by a projection lens 28. It is seen that the system is comprised of two sets of dichroic mirrors, one set (14 and 16) for color separation and another set (24 and 26) for color combination, two reflection mirrors 30 and 32 respectively associated with the separating and combining sets of dichroic mirrors and three LC panels 18, 20 and 22.
However, the light efficiency of such LCD projectors is rather limited (about one lumen per watt) and their maximum light output is also quite limited (about fifty lumens per square inch of liquid crystal), primarily because a large fraction of the incident light (up to about 70%) is intercepted by the transistors and matrix-addressing lines embodied in the TFT-LCD panel. This not only reduces the efficiency, but also causes overheating of the matrix; the authors of the article first listed above acknowledge that the most significant problem remaining in projectors that use LCD panels is suppressing high panel temperatures. Although projectors that use LC panels are usually designed to eliminate harmful infra-red and ultraviolet radiation emitted by the source, the dissipation of heat caused by the absorption in the panel of visible light is a basic limiting factor. Picture contrast ratio decreases as the temperature of the panel increases, and long-term use under these conditions eventually destroys the panel. The limited temperature the panels can tolerate, and the limited heat dissipation of the panel, limits the maximum light output and, accordingly, limits the brightness and quality of the projected picture.
Another factor which adversely affects the efficiency of projectors utilizing LC panels is the need to polarize the light; this reduces the light available by a minimum of a factor of two, with the unused light being absorbed by the polarizer. While it would appear that this problem could be obviated by using liquid crystal materials which do not require polarized light, such as polymer dispersed LC material, the scattering mode of such materials cannot at the same time achieve the high brightness and high contrast ratio necessary for projection of high quality pictures. High contrast can only be obtained by using a small aperture projection lens. Since a large aperture projection lens is required for high brightness, use of polymer dispersed LC material would limit the total light flux to an unacceptably low level.
It is a primary object of the present invention to provide an improved projector using liquid crystal modules as light-valves, which has improved efficiency and light output as compared to currently available projectors of this type.
It is a further object of the invention to provide a color image projection system which has good contrast ratio and good efficiency at the same time with a large aperture projection lens.
A further object of the invention is to provide a color projection system that uses fewer dichroic mirrors and fewer LC panels than available projectors of this type and, therefore, is simpler and less costly to manufacture.